The present invention relates generally to color optical scanners and, more particularly to variable speed, single pass color optical scanners which employ three line optical sensor arrays.
Color optical scanners are similar to black and white optical scanners in that data representative of a scanned document (object) is produced by projecting an image of the scanned document onto optical sensors. The optical sensors produce data signals representative of the intensity of the light impinged thereon. These data signals are typically digitized and stored on appropriate data storage media. Such stored data may later be used, as for example through a personal computer and computer monitor, to produce a display image of the scanned object. The image of the scanned object is projected onto the optical photosensor array incrementally by use of a moving scan line. The moving scan line is produced either by moving the document with respect to the scanner optical assembly or by moving the scanner optical assembly relative to the document.
Color optical scanners differ from black and white scanners in that multiple color component images of an object must be collected and stored to produce a color display image of the object. Typically data representative of red, green and blue component color images of the scanned object are produced and correlated for storage.
Various techniques are used in color optical scanners for collecting data representative of multiple component color images. One technique is to project imaging light onto a single linear sensor array during multiple scanning passes using differently colored illumination sources. For example a document is first scanned using only red light, then only green light and finally only blue light. In a variation of this technique three scanning passes are made using a white light illumination source but the imaging light is filtered before it enters the sensor array with a different color filter during each of the three passes.
Another technique, such as described in Vincent, U.S. Pat. No. 4,709,144 and Boyd, et al., U.S. Pat. No. 4,926,041, which are both hereby specifically incorporated by reference for all that is disclosed therein, is to split a polychromatic scan line light beam into multiple color component beams which are projected onto multiple linear photosensor arrays. For example an imaging beam from the same narrow scan line region of a document is split into red, green and blue component beams which are then simultaneously projected onto separate linear photosensor arrays. Using this technique the component color image data generated from any particular scan line is generated simultaneously and is thus easily stored in a correlated form.
Yet another technique for generating multiple color component images from a polychromatic light beam is to simultaneously project light from different scan line regions of a document onto separate linear photosensor arrays such as described in Takeuchi, R. et al. (1986) "Color Image Scanner with an RGB Linear Image Sensor," SPSE Conference, The Third International Congress On Advances in Non-Impact Printing Technologies, PP339-346, August 1986, which is hereby specifically incorporated by reference for all that it discloses. Using this technique it is necessary to perform data manipulation to correlate the data representative of different scan line component images since the different component color images of any scan line region of the document are generated at different times.
Various types of photosensor devices may be used in optical scanners. Currently the most commonly used photosensor device for optical scanners is the charge coupled photosensor device or "CCD". A CCD builds up an electrical charge in response to exposure to light. The size of the electrical charge built up is dependent on the intensity and the duration of the light exposure.
In optical scanners CCD cells are aligned in linear arrays. Each cell or "pixel" has a portion of a scan line image impinged thereon as the scan line sweeps across the scanned object. The charge built up in each of the pixels is measured and then discharged at regular "sampling intervals". In most modern optical scanners the sampling intervals of the CCD arrays are fixed. A typical CCD sampling interval is 4.5 milliseconds.
As previously mentioned an image of a scan line portion of a document is projected onto the scanners linear sensor array by scanner optics. The scanner optics comprise an imaging lens which typically reduces the size of the projected image from the original size of the document considerably, e.g. by a ratio of 7.9:1. Pixels in a scanner linear photosensor array are aligned in a "cross" direction, i.e. a direction parallel to the longitudinal axis of the scan line image which is projected thereon and perpendicular to the direction of movement of the scan line across the object. The direction perpendicular to the "cross" direction and parallel to scan line movement on the object will be referred to herein as the "scan" direction. Each pixel has a "length" measured in the cross direction and a "width" measured in the scan direction. In most CCD arrays the length and width of the pixels are equal, e.g. 8 microns in each dimension. The "line width" of a linear CCD array is the same as the width of each of the individual pixels in the array.
At any instant when an object is being scanned, each pixel in the CCD array has a corresponding area on the object which is being imaged thereon. This corresponding area on the scanned object will be referred to herein as a "native object pixel" or "native pixel". A native object pixel has dimensions equal to the dimensions of the corresponding pixel on the linear photosensor array multiplied by the magnification ratio of the scanner imaging lens.
Scanners are typically operated at a scan line sweep rate such that one native object pixel width is traversed during each CCD sampling interval. However it has been discovered, as disclosed in Meyer et al., U.S. Pat. No. 5,047,871 which is hereby specifically incorporated by reference for all that it discloses, that the resolution of a display image produced with data generated by some scanners, and thus the size of the display image, may be controlled by controlling the scan line sweep rate of the scanner. For example, by increasing the scan line sweep speed from one scan line per CCD interval to two scan lines per CCD interval, the CCD "sees" two scan line widths of the scanned document during a single sensing interval. As a result, a display image produced from the CCD data signal of the faster scan speed is one-half the size of a display image produced from a CCD signal generated at the slower scan speed. Describing this phenomena in different words, the increased scan speed results in an effective increase in the width of object pixels. A scan speed of two scan lines per CCD interval results in " effective object pixels" which are twice the width of native object pixels. The ability to "scale" the image produced by a display device by controlling scanner sweep speed is a significant feature which is offered on many newer scanners.
The color optical scanner described in the Takeuchi paper operates much more quickly than multiple pass color optical scanners. Sizing of display images was achieved through use of a zoom lens assembly. However, such a lens assembly adds considerably to the cost of an optical scanner. If the scanner of the Takeuchi paper were operated at scanning speeds different from the one set speed of the scanner in an attempt to achieve image scaling, the resulting data would be scrambled and unusable.